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What matters most is total watch time. Getting 50% retention on a 10 minute video is better than getting 99% retention on a 5 minute video. Retention is great, but it's meaningless if it doesn't ultimately extend your total watch time across your channel. That being said, a high retention still in a way signifies that your content is interesting and worthy of the viewer's time to sit through as much as they can. One thing to analyze in retention are the timestamps in the video that has the highest drop off. These are you markers identifying points where viewers lose interest. You can decide to try and figure out why that's the case then fix it, or place a card/annotation around that point to lead them into another video. Again, the goal is to not only keep people watching your video as long as you can, but also keep people on your channel as long as you can.